When W3C's XHTML 2.0 stalled WHATWG created a new "living standard" (which is subject to constant modifications and additions) to continue HTML development. W3C attempted to create fixed standards from the document (e.g. HTML 5.1, HTML 5.2, etc) that only included things that were widely supported and that met standards for accessibility and so on. However, due to the constant churn of WHATWG's document this was difficult and WHATWG were strongly against having what they saw as competing standards.

Therefore there was some conflict between the two groups until WHATWG emerged victorious. Here's the final agreement: https://www.w3.org/2019/04/WHATWG-W3C-MOU.html

It boils down to complete capitulation by W3C.